Boundary Energy Research Articles

A multiscale ion diffusion framework sheds light on the diffusion-stability-hysteresis nexus in metal halide perovskites.

Stability and current-voltage hysteresis stand as major obstacles to the commercialization of metal halide perovskites. Both phenomena have been associated with ion migration, with anecdotal evidence that stable devices yield low hysteresis. However, the underlying mechanisms of the complex stability-hysteresis link remain elusive. Here we present a multiscale diffusion framework that describes vacancy-mediated halide diffusion in polycrystalline metal halide perovskites, differentiating fast grain boundary diffusivity from volume diffusivity that is two to four orders of magnitude slower. Our results reveal an inverse relationship between the activation energies of grain boundary and volume diffusions, such that stable metal halide perovskites exhibiting smaller volume diffusivities are associated with larger grain boundary diffusivities and reduced hysteresis. The elucidation of multiscale halide diffusion in metal halide perovskites reveals complex inner couplings between ion migration in the volume of grains versus grain boundaries, which in turn can predict the stability and hysteresis of metal halide perovskites, providing a clearer path to addressing the outstanding challenges of the field.

Nanofluid past a continuous stretching Riga sheet by Gyarmati’s principle

The work on steady laminar forced convection flow of electro magneto-hydrodynamic nanofluid over a uniformly moving sheet has been considered with the aim of evaluating non-equilibrium governing boundary layer momentum, energy and concentration equations using a classical thermodynamic variational technique. The flow, heat and mass transfer of a continuously accelerated sheet extruded in a low electrically conducting fluid, enhanced by induced ponderomotive force and diluted suspension of nanoparticles is the novel intention of the present study. The integral forms of the Lagrangian functional are constructed by determining the dual flow fields inside and on the boundary layer. Then, the necessary conditions for extremum of integral principle are derived as simple algebraic expressions in terms of boundary layer thicknesses. The thermo-physical quantities of the research interest are determined explicitly as polynomial expressions. This study examines how the skin friction coefficient, local heat and mass transfer are affected by electromagnetic force (QH), Joule heating (Ec), thermophoresis (NT) and Brownian motion (NB). The computed results indicate that electromagnetic force increases the velocity, and reduces the temperature. The concentration is increased by thermophoresis effect and decreased by Brownian diffusion. To validate the efficiency of solution procedure and confirm accuracy, certain specific results are compared with the solutions by other numerical methods available in the literature. The precision is confirmed.

Propagation of plane waves in a nonlocal transversely isotropic thermoelastic medium with voids and rotation

AbstractThe objective of present research work is to investigate the propagation of time harmonic plane waves in a rotating, nonlocal, transversely isotropic thermoelastic half‐space having void pores in the context of Lord–Shulman theory. It has been observed that four coupled plane waves travel in such type of medium with distinct speeds. Using appropriate boundary conditions, the amplitude and energy ratios for the reflected waves are presented. Numerical computations for a magnesium crystal‐like material are performed with the help of MATLAB software to study the effects of material anisotropy, nonlocal, void and rotation parameters on the amplitude ratios. The computational results are displayed and explained through graphs. The phase velocities of plane waves in transversely isotropic medium are found to depend upon the incident angle, whereas these attain constant values in an isotropic medium. The energy conservation law is also justified during the reflection phenomena.

A Co-rich chemically complex intermetallic alloy with extraordinary strength-ductility synergy

High-strength ordered intermetallic alloys are always sought-after in sophisticated structural applications. However, their practical applications have long been hindered by the severe grain-boundary embrittlement. Here, we designed a Co-rich chemically complex intermetallic alloy (CCIMA) with the L12-type ordered structure, which exhibits an ultrahigh tensile strength of ∼1,611 MPa and a substantial ductility of ∼37%. These exceptional strength-ductility combination outperforms the majority of common Co-rich intermetallic alloys. The multi-addition of Ta, Ti, and V elements is largely responsible for the significantly increased antiphase boundary energy of the L12 superlattice, which accounts for the high strength. This high degree of ductility is attributed to both the increased fracture resistance of grain boundaries brought about by co-segregation-induced order-to-disorder phase transformation, as well as the high work-hardening capability.

Universality of grain boundary phases in fcc metals: Case study on high-angle [111] symmetric tilt grain boundaries

Grain boundaries often exhibit ordered atomic structures. Increasing amounts of evidence have been provided by transmission electron microscopy and atomistic computer simulations that different stable and metastable grain boundary structures can occur. Meanwhile, theories to treat them thermodynamically as grain boundary phases have been developed. Whereas atomic structures were identified at particular grain boundaries for particular materials, it remains an open question if these structures and their thermodynamic excess properties are material specific or generalizable to, e.g., all fcc metals. In order to elucidate that question, we use atomistic simulations with classical interatomic potentials to investigate a range of high-angle [111] symmetric tilt grain boundaries in Ni, Cu, Pd, Ag, Au, Al, and Pb. We could indeed find two families of grain boundary phases in all of the investigated grain boundaries, which cover most of the standard fcc materials. Where possible, we compared the atomic structures to atomic-resolution electron microscopy images and found that the structures match. This poses the question if the grain boundary phases are simply the result of sphere-packing geometry or if material-specific bonding physics play a role. We tested this using simple model pair potentials and found that medium-ranged interactions are required to reproduce the atomic structures, while the more realistic material models mostly affect the grain boundary (free) energy. In addition to the structural investigation, we also report the thermodynamic excess properties of the grain boundaries, explore how they influence the thermodynamic stability of the grain boundary phases, and detail the commonalities and differences between the materials.

Automated determination of grain boundary energy and potential-dependence using the OpenKIM framework

We present a systematic methodology, built within the Open Knowledgebase of Interatomic Models (OpenKIM) framework (https://openkim.org), for quantifying properties of grain boundaries (GBs) for arbitrary interatomic potentials (IPs), GB character, and lattice structure and species. The framework currently generates results for symmetric tilt GBs in cubic materials, but can be readily extended to other types of boundaries. In this paper, GB energy data are presented that were generated automatically for Al, Ni, Cu, Fe, and Mo with 225 IPs; the system is installed on openkim.org and will continue to generate results for all new IPs uploaded to OpenKIM. The results from the atomistic calculations are compared to the lattice matching model, which is a semi-analytic geometric model for approximating GB energy. It is determined that the energy predicted by all IPs (that are stable for the given boundary type) correlate closely with the energy from the model, up to a multiplicative factor. It thus is concluded that the qualitative form of the GB energy versus tilt angle is dominated more by geometry than the choice of IP, but that the IP can strongly affect the energy level. The spread in GB energy predictions across the ensemble of IPs in OpenKIM provides a measure of uncertainty for GB energy predictions by classical IPs.

Moiré patterns and inversion boundaries in graphene/hexagonal boron nitride bilayers

In this paper a systematic examination of graphene/hexagonal boron nitride (g/hBN) bilayers is presented, through a recently developed two-dimensional phase field crystal model that incorporates out-of-plane deformations. The system parameters are determined by closely matching the stacking energies and heights of g/hBN bilayers to those obtained from existing quantum-mechanical density functional theory calculations. Out-of-plane deformations are shown to reduce the energies of inversion domain boundaries in hBN, and the coupling between graphene and hBN layers leads to a bilayer defect configuration consisting of an inversion boundary in hBN and a domain wall in graphene. Simulations of twisted bilayers reveal the structure, energy, and elastic properties of the corresponding moir\'e patterns and show a crossover as the misorientation angle between the layers increases from a well-defined hexagonal network of domain boundaries and junctions to smeared-out patterns. The transition occurs when the thickness of domain walls approaches the size of the moir\'e patterns and coincides with the peaks in the average von Mises and volumetric stresses of the bilayer.

Achieving grain refinement of α-Al and Si modification simultaneously by La‒B‒Sr addition in Al‒10Si alloys

As-cast Al‒Si alloys always face low ductility because of coarse α-Al grains and large lamellar eutectic Si. Although B-containing refiners and Sr modifiers could refine the α-Al grains and eutectic Si respectively, simultaneous addition leads to an invalid effect of Sr because B-containing refiner has a poisoning effect (PE) on Sr modifier in the Al‒10Si‒B‒Sr alloys. The present work achieves both refinement of α-Al grains and modification of Si merely by the addition of 500 ppm La in Al‒10Si‒0.02B‒0.015Sr alloy, which greatly enhances the ductility and tensile strength. Sr‒B interaction in the Al‒10Si‒0.02B‒0.015Sr melt results in a consumption of Sr by forming Sr‒B intermetallic compound. CALPHAD-type calculations reveal that Sr concentration in liquid before eutectic reaction becomes a key parameter to estimate the alloying modification effect. The addition of La forms LaB6 and thereby releases Sr to protect the Sr modification effect. First-principles calculations illustrate that both Sr and La are beneficial to the formation of twin boundaries in Si particles due to their negative formation energy of twin boundaries. The formed LaB6 has a semicoherent interface with α-Al with the orientation relationship of [11¯0]LaB6//[11¯0]Al and (11¯1)LaB6//(11¯1)Al, demonstrating that LaB6 is an efficient nucleation site for α-Al, which contributes to the refinement of α-Al in the Al‒10Si‒0.05La‒0.02B‒0.015Sr alloy. Our findings reveal the micro-mechanism of refinement during the solidification process and develop an effective and simple way to obtain high-performance Al alloys.

Precipitation characteristics of γ' precipitates of the GH4742 nickel-based superalloy at a slow cooling rate

The double cone samples of the predeformed GH4742 superalloy were treated by subsolid solution heat treatment (Sub-SHT) at 1080 ℃/8 h and then cooled at a cooling rate of 22 ℃/min. The evolution of γ' precipitates by coupling hot deformation and heat treatment was studied by in situ electron backscatter diffraction + scanning electron microscopy. This research explains the morphological variations between the recrystallized and deformed grains of the γ' precipitates, as well as the relationship between the characteristics of the grain boundaries (GBs) and γ' precipitates at the GBs. The results show that when the predeformed samples were subjected to an 8 h Sub-SHT and then cooled at 22 ℃/min, the γ' precipitates exhibit a tri-modal size distribution, and the primary γ' precipitates are dendritic. The morphology of γ' precipitates within deformed grains is related to the accumulation of dislocations and other defects. The maximum size of columnar γ' precipitates is obtained at the high-angle grain boundary, where the misorientation ranges from 30° to 40°. The length of columnar γ' precipitates results from the comprehensive effect of the GB energy determined by the GB misorientation and the GB mobility determined by the stored energy difference. Owing to the low energy and low mobility of the Σ3 twin boundary, the γ' precipitates at the Σ3 twin boundaries remain stable, and the morphology and size are no different from those within the grains. These findings may be helpful for the design and optimization of Ni-base superalloys.

Influence of group IV element on basic mechanical properties of BCC medium-entropy alloys using machine-learning potentials

In order to elucidate the origin of excellent mechanical properties of high-entropy alloys (HEA), it is essential to develop the atomic-level depiction of defect structures, taking into account the influence of composition. Especially in body-centered cubic (BCC) HEA an alteration of constituent elements may lead to a dramatic change in the behavior on the macroscopic level. To study that effect, we employed a machine learning technique and constructed highly accurate robust potentials for two BCC medium-entropy alloys: MoNbTa and ZrNbTa. Even though they have close composition, the mechanical properties of the two alloys differ not only quantitatively, but also qualitatively. We show that the group IV element Zr decreases values of bulk and elastic constants. The influence of short-range order on stacking fault and twin boundary energies is discussed. Also, we show the difference in the screw dislocation core shapes between the two alloys. The cores of 〈111〉 screw dislocations in the ZrNbTa case demonstrate a non-compact shape substantially extended on the (110) plane.

On p-Willmore disks with boundary energies

We consider an energy functional on surface immersions which includes contributions from both boundary and interior. Inspired by physical examples, the boundary is modeled as the center line of a generalized Kirchhoff elastic rod, while the interior term is arbitrarily dependent on the mean curvature and linearly dependent on the Gaussian curvature. We study equilibrium configurations for this energy in general among topological disks, as well as specifically for the class of examples known as p-Willmore energies.

Effect of extrusion process on the stress corrosion cracking resistance of 7N01 aluminum alloy

In this work, the effect of extrusion process on the mechanical properties and the stress corrosion cracking (SCC) resistance of 7N01 aluminum alloys were systematically investigated by tensile testing, slow strain rate testing (SSRT), electrochemical experiment, scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), and transmission electron microscope (TEM) observation, respectively. The results showed that with the increase of extrusion temperature, the SCC resistances of the alloys deteriorated, and this tendency was also proved by the electrochemical experiments including polarization curves and EIS results. The Microstructure observation results revealed that recrystallization played an important role on the SCC resistances of the alloys: the new recrystallization grain boundaries with higher grain boundary energy and wider PFZ could magnify the difference of electrochemical property between grain boundary area and the grain interior, thus raising the stress corrosion crack sensitivity of the alloys.

Microstructure evolution induced by solidification and ferrite–austenite massive-like transformation in Fe–C alloys

The coarsening of austenite grains produced through a massive-like transformation from the ferrite to the austenite phase during/after solidification in Fe–0.45mass%C steel was examined through time-resolved X-ray diffraction measurements using time-resolved computed tomography. The δ phase solidification resulted in a single δ grain in the observation region (0.6 mm in diameter, 0.1 mm in height). The massive-like transformation produced multiple γ grains, and coarsening immediately followed the transformation. The coarsening in the observation region was completed even within 200 s. A correlation of crystallographic orientation between the γ grains was found in the γ grain structure. The massive-like transformation frequently produced fcc (111) twinned grains, and the twinned austenite grains tended to remain during coarsening because the twin interface energy is lower than the grain boundary energy. The massive-like transformation following the solidification contributed to the texture formation in the γ grain structure.

QUANTITATIVE CHARACTERIZATION OF GRAIN BOUNDARIES IN ULTRAFINE-GRAINED AUSTENITIC STAINLESS STEEL BY CLUSTER ANALYSIS

A method is proposed for analyzing the relative energy distributions of boundaries in ultrafine-grained materials measured by grain boundary grooving using scanning tunneling microscopy. The energy distribution of boundaries in a grain boundary ensemble is considered as a superposition of individual distributions (populations), which can be identified by cluster analysis based on statistical criteria and each of which has its own value of average energy, dispersion, and share in the overall distribution. The energy distributions of grain boundaries are analyzed for 12Cr15Mn9NiCu steel with a coarse-grained structure in the as-received state and with an ultrafine-grained structure produced by hot helical rolling and subsequent cold rolling. It is shown that the number of boundary populations and their main characteristics revealed by cluster analysis depend on the steel structure. The results of cluster analysis of experimental distributions are compared with the EBSD measurement data on grain boundary misorientation distributions. Discrepancy between the cluster analysis results for the energy and misorientation distributions of boundaries is discussed taking into account the difference in the nature of the data obtained.

Understanding Meteorological Changes Following Severe Defoliation during a Strong Hurricane Landfall: Insights from Hurricane Michael (2018)

Abstract Despite prompting persistent meteorological changes, severe defoliation following a tropical cyclone (TC) landfall has received relatively little attention and is largely overlooked within hurricane preparedness and recovery planning. Changes to near-track vegetation can modify evapotranspiration for months after tropical cyclone passage, thereby altering boundary layer moisture and energy fluxes that drive the local water cycle. This study seeks to understand potential spatial and temporal changes in defoliation-driven meteorological conditions using Hurricane Michael (2018) as a testbed. In this sensitivity study, two Weather Research and Forecasting (WRF) Model simulations, a normal-landscape and a post-TC scenario, are compared to determine how a defoliation scar placed along Michael’s path alters surface heat fluxes, temperature, relative humidity, and precipitation near the storm’s track. In the month following the foliage reduction, WRF resolves a 0.7°C 2-m temperature increase, with the greatest changes occurring at night. Meanwhile, the simulations produce changes to the sensible and latent heat fluxes of +8.3 and −13.9 W m−2, respectively, while average relative humidity decreases from 73% to 70.1%. Although the accumulated precipitation in the defoliated simulation was larger along a narrow corridor paralleling and downwind of the hurricane track, neither simulation satisfactorily replicated post-Michael precipitation patterns as recorded by NCEP Stage IV QPE, casting doubt as to whether the downwind enhancement was exclusively due to the defoliation scar. This sensitivity analysis provides insight into the types of changes that may be possible following rapid and widespread defoliation during a TC landfall.

Microstructural and mechanical properties of non-equimolar NiCoFeAlTi medium entropy alloy strengthening by L12 nanoparticles

Ideal structural materials usually require excellent mechanical properties, high entropy alloys/medium entropy alloys (HEAs/MEAs) are considered to be the key material to achieve this goal. In the current work, a novel L12 precipitation strengthened medium entropy alloy is designed. After rolling and recrystallization, the alloy which is then aged possesses a mechanical property of a yield strength of 1.09 GPa, an ultimate tensile strength of 1.5 GPa and nearly 20% elongation at room temperature (the as-aged alloy). However, the plasticity of the as-aged alloy which is fully recrystallized is poor at high temperature. Then, the non-recrystallized alloy, which is directly aged after rolling, containing coarse lengthening grain with lower grain boundary energy, exhibits excellent high temperature mechanical properties.

Tachyon-dependent Chern-Simons terms and the V-QCD baryon

The structure of the five-dimensional Tachyon-Chern-Simons action and its relevance to single-baryon states in the context of the V-QCD models for holographic QCD with backreacting flavor are analyzed. The most general form of the Tachyon-Chern-Simons 5-form, compatible with symmetries and flavor anomalies is determined. It is the sum of a non-trivial gauge-invariant 5-dimensional form and a non-invariant closed 5-form that reproduces the flavor anomalies. Single-baryon solutions of the gravity theory, arising from the DBI plus Tachyon-Chern-Simons actions are considered. The baryon is realised as a bulk axial instanton. The baryon ansatz and the field equations are derived and the boundary conditions are determined, which ensure that the solution has finite boundary energy and unit baryon charge. The boundary baryon number, which is computed from the universal (closed) part of the Tachyon-Chern-Simons action, is shown to coincide with the bulk axial instanton number.

Enhanced thermal stability and mechanical properties of the nanocrystalline multicomponent Fe–(ZrNbMoTa)x alloys

Thermal stability and mechanical property of the multicomponent nanocrystalline Fe–ZrNbMoTa alloys were investigated. Single-phase nanocrystalline Fe–(ZrNbMoTa)x (x = 0.1, 0.2, 0.5 and 1 at. %) alloys were prepared by ball milling and followed by annealing in a set of predesigned condition. It was found that the Fe-based multicomponent alloys possess good thermal stability through 10 h annealing at 900 °C, which is attributed to the thermodynamics effect in the reduction of grain boundary energy caused by multicomponent co-segregation of Zr, Nb, Mo and Ta, and kinetics effect related to sluggish diffusion and Zener pinning of a small amount of second-phase precipitation. Additionally, nanocrystalline Fe–(ZrNbMoTa)x alloys show excellent mechanical properties. The hardness is 738.6 HV, and its yield strength and compressive strength reach up to 3100 MPa and 3600 MPa, respectively. These are attributed to the comprehensive effects of nanocrystalline, solid solution and second phase strengthening.

Mechanical Properties and Energy Characteristics of Flawed Samples with Two Non-Parallel Flaws under Uniaxial Compression

Unequal flaws are widespread in rock engineering, in which the extensions induced by excavation and unloading are the main factors leading to engineering instability. Rock deformation and failure are essentially the results of energy transmission. In order to study the influence of flaw type and angle on the mechanical properties and energy characteristics of the flawed rock mass, uniaxial compression tests were conducted using particle flow numerical analysis software. The results show that the crack initiation strength and peak strength of the sample increase with the increase in the flaw angle. For smaller flaw angles, the sample shows obvious plastic deformation during uniaxial compression. For larger flaw angles, the sample shows elastic–brittle properties, whose storage energy is larger. The peak strength of samples with flaws of unequal length presents an obvious decrease compared with that of flawed samples with flaws of equal length, and the extent of the reduction is 6.15% on average. Unequal flaws decrease the ability of the flawed sample to absorb strain energy. Compared with flawed samples with equal flaws, flawed samples with unequal flaws decrease the boundary energy and elastic strain energy by 9.29% and 9.95% on average, respectively. Flaws of unequal length in this sample can weaken the performance of the flawed sample.

A Mechanism of Double-Membrane Vesicle Formation from Liquid-Ordered/Liquid-Disordered Phase Separated Spherical Membrane.

Genome replication of coronaviruses takes place in specific cellular compartments, in so-called double-membrane vesicles (DMVs), formed from the endoplasmic reticulum (ER). An intensive production of DMVs is induced by non-structural viral proteins. Here, we proposed a possible mechanism of the DMV formation from ER-derived spherical vesicles where liquid-ordered and liquid-disordered lipid phases coexist. These vesicles are supposed to divide into two homogeneous liquid-ordered and liquid-disordered vesicles. The formation of two spherical vesicles constituting DMV requires a mechanical work to be performed. We considered the excess energy of the boundary between the coexisting lipid phases as the main driving force behind the division of the initial vesicle. Explicitly accounting for the energy of elastic deformations and the interphase boundary energy, we analyzed a range of physical parameters where the DMV formation is possible. We concluded that this process can principally take place in a very narrow range of system parameters. The most probable diameter of DMVs formed according to the proposed mechanism appeared to be approximately 220 nm, in an agreement with the average diameter of DMVs observed in vivo. Our consideration predicts the DMV size to be strongly limited from above. The developed analysis can be utilized for the production of DMVs in model systems.